1. Field of the Invention
The present invention relates to an optical information recording medium and, more in particular, it relates to a write-once optical information recording medium by heat mode.
2. Description of the Related Art
Optical information recording media (optical disks) capable of recording information for once using laser light have been conventionally known. The optical disks are also referred to as write-once CDs (so-called CD-Rs) and in a typical structure thereof, a recording layer containing an organic dye, a reflection layer containing a metal such as gold and a protective layer (cover layer) made of a resin are laminated successively in this order on a transparent disk-shape substrate. Information is recorded to the CD-R by irradiating the CD-R with a laser light in a near infrared region (usually, a laser light at a wavelength near 780 nm). An irradiated area of the recording layer absorbs the light to locally increase the temperature, and the optical characteristics of the area are changed by physical or chemical change (for example, formation of pits) to record the information. On the other hand, reading (playback) of information is also conducted by irradiating the CD-R with a laser light at wavelength identical to that of the recording laser light. Reading of information is conducted by detecting the difference of the reflectance between an area where the optical characteristics have changed (recorded area) and an area where they have not changed (non-recorded area) in the recording layer.
In recent years, information recording media of higher recording density are being demanded. In response to such a demand, an optical disk referred to as a write-once digital versatile disk (so-called DVD-Rs) has been proposed (for example, in “Nikkei New Media” Special volume “DVD”, issued in 1995). The DVD-R has a structure comprising two disks each formed usually by laminating a recording layer containing an organic dye, a reflection layer and a protective layer in this order on a transparent disk-shape substrate in which guide grooves (pre-grooves) for tracking of laser light to be irradiated are formed at a narrow groove width (0.74-0.8 μm) of one-half or less a groove width of CD-Rs, which two disks are adhered to each other with the recording layers disposed at the inside, or a structure of adhering the disk described above and a disk-shaped protective substrate of the same shape as the disk with the recording layer disposed at the inside. Information is recorded to and played back from the DVD-R by irradiating the DVD-R with a visible laser light (usually, laser light at a wavelength in a region of 630 nm to 680 nm), and recording at higher density than CD-R is possible.
In recent years, networks such as the Internet and high vision TV have become popular rapidly and broadcasting of high definition television (HDTV) will also be started soon. Under the circumstances, optical information recording media of large capacity capable of recording image information conveniently at a reduced cost are required. While DVD-R can sufficiently perform a role as optical information recording media of large capacity at present, demands for larger capacity and higher density have increased more and more and development of optical information recording media capable of coping with such requirement is also necessary. Accordingly, optical information recording media of larger capacity capable of recording at high density by the light of a shorter wavelength compared with that of DVD-R have been under development.
For example, an optical information recording medium comprising, on a substrate, a metal reflection layer, a recording layer and a cover layer, which is thinner than the substrate, stacked in this order on the substrate based on the premise of recording by light transmitted through a lens with high NA, which is suitable to high density recording, has been disclosed. In addition, recording and playback methods of recording and playing back information by being irradiated with laser light at a wavelength of 550 nm or less from the side of the recording layer to the side of the reflection layer have been disclosed. As a dye contained in the recording layer of the optical information recording medium utilizing such short wave laser light, porphyrine compounds, azo dyes, metal azo dyes, quinophthalone dyes, trimethincyanine dyes, dicyanovinylphenyl skeleton dyes, and cumarin compounds are proposed, for example, in JP-A Nos. 4-74690, 7-304256, 7-304257, 8-1271705, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-149320, 2000-158818, 2000-228028, and 2001-146074). It will be obvious to those skilled in the art that other compounds than described above can also be used, in principle, as dyes for recording layer so long as they are compounds capable of absorbing laser light used for recording. For the laser light for recording and playback, blue-purple color (wavelength at 405 nm or 410 nm), blue color (wavelength at 430 nm or 488 nm), and blue-green color (wavelength at 515 nm) have been proposed.
The optical information recording medium capable of recording and playing back information by the laser light at a short wavelength described above generally has, for example, a reflection layer, a recording layer, a barrier layer, a pressure sensitive adhesive layer or an adhesive layer, and a cover sheet in this order on a substrate, which is different from CD-Rs or DVD-Rs in view of the layer constitution. Accordingly, the irradiation direction of the laser light is opposite to that in CD-Rs or DVD-Rs and problems sometimes occur with peeling of layers, which was not a problem in CD-Rs or DVD-Rs and strong adhesion is required between each of the layers. However, since a boundary liable to be peeled is present in view of the layer constitution in which an organic material (recording layer, etc.) and inorganic material (reflection layer, barrier layer, etc.) in adjacent with each other, sufficient bonding strength sometimes cannot be obtained, leaving room for improvement.
Further, in the optical information recording medium of the layer constitution described previously, the recording layer is externally exposed, particularly at the outer circumferential edge, so that the storability worsens due to atmospheric temperature and humidity.